1. Description of Prior Art and Related Information
The present invention generally refers to an overcurrent protection device for electrical circuits and to the use of such device on protection elements for electrical circuits, such as fuse links, associated limiter fuses and semiconductor protection fuses.
The protection of electrical devices that require fuses for small nominal currents up to some tens of amps, entails the problem of the unexpected and premature blowing of the fuse as a result of the presence of transient currents in the circuit, originated by events such as lightning strikes, opening and closing of circuits and electrostatic discharges. These events may generate, by electrical induction, overcurrents that may seriously damage the circuits, unless they are conveniently protected.
Impulse currents flowing through the electricity lines are catalogued according to international standards, and they may be broken down in three basic categories, namely lightning, steep and switching impulse currents. The most significant type, as far as premature fuse blowing is concerned, are the lightning-type impulse currents, originated as a result of the discharge of a lightning arrester after a lightning strike. Such impulses have a waveform of 8/20 μs, i.e., 8 μs of rise and 20 μs of fall, up to the half value of the queue length. As far as current width is concerned, these are usually peak values ranging from 5 kA to 10 kA for high Voltages and from 1 kA to 5 kA for secondary low voltage facilities.
To protect circuits against overvoltages, varistors or lightning arresters are mainly used. These devices earth the electrically induced energy, so that the voltage level reached by the equipment does not become dangerous. When this earth leak occurs, a transient current is generated, which crosses the whole circuit, so that if such transient current crosses a fuse, this might become blown.
Different solutions have been historically suggested to solve this problem. The first one consists of installing the lightning arrester upstream from the fuse, so that the current eventually leaked from the lightning arrester does not cross the said fuse. This solution became obsolete and is no longer actually used, since the protection margin afforded by the lightning arrester decreases with distance. This means that a really effective overvoltage protection must be installed at the connection terminals of the item to be protected, as close as possible to such item, both from a physical and from an electrical point of view.
For this reason, fuses are also being installed inside the transformer tank, which usually poses the problem that, in case that the fuse is blown, it is necessary to replace the whole transformer, on the assumption that if the fuse is blown, this means that the transformer is permanently damaged, which is not always true.
A solution which has been widely used comprises the use of fuses intended for a sufficiently high current, so that they do not blow as a result of impulse currents. Thus, some facilities incorporate 50 A fuses for the protection of transformers with a rated current of 1.44 A (50 kVA at 20 kV). Obviously, these fuses do not protect the transformer at all and, in case that they must be tripped as a result of a failure of the transformer, this latter may be already completely destroyed.
Another solution consists of slowing down the operation of the fuses in short times or, in other words, in the presence of high currents, such as impulses, so that they take more time to trip, and speeding such operation up at long times. These are the so-called “dual link” fuses, such as, for instance, those mentioned in U.S. Pat. No. 4,121,187, U.S. Pat. No. 5,300,914, U.S. Pat. No. 5,119,060 and U.S. Pat. No. 5,852,396.
Apart from this, there is a problem with the protection of power semiconductors, like diodes, SCR, IGBT, etc. These devices, that are characterised by their great switching capacity and the large number of operations that they may perform, are also very sensitive to current impulses, so that they may operate millions of times under their specified normal parameters, but they may become destroyed by a single impulse current of a few milliseconds. The solution for this problem consists of using the so-called semiconductor fuses, that are basically fast fuses, with low I2t operating values. Thus, the fuse receives the impulse and stops it, even though it becomes blown. The problem with these fuses is that they are so fast that they may also blow under certain circumstances such as prior heating, impulse trains, etc.
2. Field of the Invention
On the basis of the previously described state of the art, one of the objects of the invention is to develop a protection device of the above-mentioned type, which can be easily produced and is sufficiently strong from an electricity point of view. The invention is based on the idea that the low intrinsic self-induction of a conductor connected between a current input and a current output generates a high drop of inductive voltage between such terminals, in the presence of current overshoots at the current input, as a result of the high value of changes in current along the time, taking advantage of the high voltage drop caused by the presence of current overshoots to produce, with the help of the skin effect, a discharge arc through a third terminal, hereinafter the central terminal, which has been provided between the input terminal and the output terminal, such central terminal being externally put at a voltage similar to the voltage of the output terminal, thus preventing the flow of current overshoots through the electrical conductor element and passing such current overshoots through such central terminal.
This first object is achieved through the characteristics of an overcurrent protection devices as described herein. Other characteristics and advantages will be indicated through the characteristics described herein.
The overcurrent protection device for electrical circuits is characterised, according to the invention, in that it comprises:                a current input terminal;        a central output terminal for current overshoots; and        an output terminal for the output of non-impulse currents which is outwardly connected to the voltage of the central terminal, upstream from the current overshoot at the input terminal; and        an electric conductor element connected between the current input terminal and the output terminal, such conductor showing a reduced self-induction when operating without current overshoots but, which, in case that current overshoots are present at the input terminal, produces a high drop of the induction voltage between such input terminal and the output terminal.        
Wherein the central terminal is firmly arranged in the vicinity of the current input terminal, at a distance that may be adjusted so that when, as a result of a current overshoot, the drop of induction voltage between such input terminal and the output terminal for non-impulse currents exceeds the dielectric strength of the dielectric material present between the input and the central terminals, and thanks to the skin effect, an arc is produced between said input and central terminals, preventing the flow of current overshoots through the conductor.
According to an additional characteristic of the invention, it is advantageous to insert, between the current input terminal and the output terminal, a tube made of insulating material, wherein the electrical conductor element connected between the input terminal and the output terminal is housed inside such tube.
According to another characteristic of the invention, it is advantageous, from the point of view of ease of manufacturing, that the input terminal is connected to a disc-shaped electrode coupled to one of the ends of the insulating tube, and that the output terminal is connected to a casing arranged at the other end of such insulating tube, the central terminal being connected to, or forming part of a ring-shaped electrode which has been fixed around the insulating tube.
According to yet another additional characteristic of the invention, and in order to control the discharge arc, it is advantageous that the electrode of the input terminal comprises a neck portion, and that the electrode of the central terminal also comprises a neck portion incorporating a central hole where an insulating sleeve is housed, wherein such sleeve is crossed by the electrical conductor element, and where the relevant neck portions extend within an area which is delimited by the walls of a cylindrical, gas-tight element inserted between both electrodes, defining a controlled atmosphere discharge chamber for the generation of the arc between the relevant neck portions of the electrodes, such insulating tube extending between the lower end of the electrode of the central terminal and the casing of the output terminal.
Furthermore, according to yet another characteristic of the invention, it is easier to connect the protection device by welding or crimping, provided that both the current input terminal and the output terminal protrude from the ends of the insulating tube and all the terminal ends are arranged in the shape of a shovel or tube.
Also according to another additional characteristic of the invention, it is advantageous that the dielectric strength of the insulating tube is higher than that of the surrounding medium.
According to yet another additional characteristic of the invention, the dielectric strength of the insulating tube may be adjusted if such tube is provided with holes, for instance, eliminating wall sections of such tube, at the area comprised between the input and the central terminals.
Furthermore, it may be advantageous, according to another characteristic of the invention, that the electrodes of the input and central terminals, or the neck-shaped portions of such electrodes, are connected by means of a varistor or condenser. Furthermore, it is possible to advantageously connect, between the input and the central terminals, a varistor or condenser incorporating a hole through which the conductor runs.
According to yet another additional characteristic of the invention, the electrical conductor element is advantageously provided as a conducting wire, either following a straight pattern or wound in spires, whose diameter, length and number of spires are respectively chosen depending on the rated current of the fuse and the self-induction value desired for such electrical conductor element, or providing such conductor as an actual ballast as such.
Even according to yet another additional characteristic of the invention, the generation of the discharge arc is facilitated thanks to a magnetic repulsion effect, when the insulating tube consists of a flexible material, so that it can be arranged as a wrapped configuration, with a toroidal cross-section.
Another additional object of the invention is to use the protection device by connecting it to a fuse link of a circuit. This objective is achieved by means of the characteristics described herein.
To that effect, a fuse link incorporates a connecting cap; a link header connected to the connecting cap; a first connection, which is connected to the link header; a second connection, which is connected to a third connection of a pigtail connection through a fusible element; a tensioning wire coupled between the second and the third connections; and an ancillary protection tube wherein the link header, the first and second connections, the fusible element, the tensioning wire and part of the connecting pigtail are encapsulated, and wherein the protection device against overcurrents is connected at such fuse link to the first connection of the fuse link by means of the input terminal, to the second connection of the fuse link by means of the output terminal, and to the third connection of the fuse link by means of the central terminal, through an overcurrent-resistant conducting wire.
Another additional object of the invention is the use of the protection device while it is connected to an associated limiter fuse of a circuit. Such object is achieved by means of the characteristics explained herein.
Thus, we have an associated limiter fuse respectively incorporating a first and a second connection capsules; a fusible element arranged following a spiral layout around an insulating core which extends between the connection capsules; a first contact portion connected to the first connection capsule and another contact portion, separated from the first portion, which is connected to the fusible element; a third contact portion which is adjacent to the second contact portion and which is electrically insulated from such portion by means of an insulating portion, and a fourth metal contact element which is connected to the second connection capsule, wherein the overcurrent protection device is connected to such fuse link, with the input terminal connected to the first contact portion; the output terminal, to the second contact portion, and the central terminal to the third contact portion, by means of a non-fusible conductor.
Another additional object of the invention is the use of the protection device while it is connected to a protection fuse for semiconductors. This objective is achieved by means of the characteristics indicated herein.
Thus, we have a semiconductor protection fuse including a first and a second connection capsules; and a main fusible element extending between such first and second connection capsules, wherein the overcurrent protection device is connected to such protection fuse with the input terminal connected to the first connection capsule and the output terminal connected to the main fusible element of the protection fuse, while the central terminal is connected, through a non-fusible connecting wire, to the main fusible element; and wherein, if appropriate, a protection element, such as a condenser or varistor is connected between the input terminal and the central terminal.